The present invention relates generally to ion implantation systems, and more specifically to an electrostatic clamp having a discharge path associated with a dielectric layer thereof for bleeding off charge which may accumulate during workpiece processing.
Ion implanters are used to implant or xe2x80x9cdopexe2x80x9d silicon wafers with impurities to produce n or p type extrinsic materials. The n and p type extrinsic materials are utilized in the production of semiconductor integrated circuits. As its name implies, the ion implanter dopes the silicon wafers with a selected ion species to produce the desired extrinsic material. Implanting ions generated from source materials such as antimony, arsenic or phosphorus results in n type extrinsic material wafers. If p type extrinsic material wafers are desired, ions generated with source materials such as boron, gallium or indium will be implanted.
The ion implanter includes an ion source for generating positively charged ions from ionizable source materials. The generated ions are formed into a beam and accelerated along a predetermined beam path to an implantation station. The ion implanter includes beam forming and shaping structure extending between the ion source and the implantation station. The beam forming and shaping structure maintains the ion beam and bounds an elongated interior cavity or region through which the beam passes en route to the implantation station. When operating the implanter, the interior region must be evacuated to reduce the probability of ions being deflected from the predetermined beam path as a result of collisions with air molecules.
For some types of ion implanters, the wafer or workpiece at the implantation station is mounted on a surface or support pedestal. For serial type implantation systems, a rotating support is not used and therefore a mechanism is typically employed to secure the workpiece on the support pedestal. One type of securement mechanism is an electrostatic chuck or clamp. While electrostatic chucks or clamps may vary in design, they are based primarily on the principle of applying a voltage to one or more electrodes embedded in the chuck so as to induce opposite polarity changes in the workpiece and electrode(s), respectively. The electrostatic attractive force between the opposite charges pulls the workpiece against the chuck, thereby retaining the workpiece in its position in a secure manner.
A typical electrostatic chuck or clamp includes an electrode covered by an insulator or dielectric layer. When the electrode of the chuck or clamp is electrically biased with respect to the substrate or workpiece by a voltage, an attractive electrostatic force is generated that holds the substrate or workpiece to the chuck. In monopolar electrode type chucks, an electrically charged plasma above the substrate induces electrostatic charge in the substrate that electrostatically holds the substrate to the chuck. A bipolar electrode chuck comprises bipolar electrodes that are electrically biased relative to one another to provide the electrostatic attractive force.
Referring initially to prior art FIG. 1, a simplified electrostatic chuck or clamp 10 is illustrated, wherein the chuck includes a dielectric or insulating region 12 overlying an electrode 14. A workpiece 16, for example, a silicon wafer undergoing implantation, overlies the dielectric region or cover 12. In operation, a voltage potential 18 is applied across the wafer 16 via the electrode 14. Due to the presence of the dielectric layer 12 which exhibits a large electrical resistance, an accumulation of electrostatic charge in the wafer 16 and the electrode 14 results in a coulombic electrostatic force characterized by the equation:
F=(xc2xd)xcex5oxcex5rA(V/t)2,
wherein xcex5o and xcex5r are the dielectric constants associated with a vacuum and the dielectric layer 12, respectively, A is the area of the electrode, V is the voltage applied to the electrode 14 via the source 18, and t is the thickness of the dielectric layer 12.
Another type of electrostatic clamp or chuck (not shown) employs Johnsen-Rahbek electrostatic attraction forces, which are a function of charge accumulation across an interfacial contact resistance such as an air gap. In any event, regardless of the particular type of clamp or chuck employed within the system, electrostatic forces work to secure the wafer in position on the chuck without need of a mechanical or physical mechanism touching the workpiece. The lack of physical clamping advantageously reduces particulate frontside contamination which may potentially result when a mechanical clamp mechanism contacts a workpiece.
One problem associated with electrostatic chucks or clamps such as the chuck 10 of prior art FIG. 1 is caused by positive charge accumulation on the workpiece 16 during implantation. As stated above, since positively charged ions are typically employed to dope a workpiece, positive charge may accumulate thereon, for example, as illustrated in prior art FIG. 2, and designated at reference numeral 20. As the charge 20 accumulates, it naturally seeks a path to ground, and when such a path is identified, such a substantial charge accumulation may result in an arcing to ground in an uncontrolled manner, which undesirably may damage the workpiece 16. Therefore mechanisms have been developed to address the problem by attempting to discharge any accumulated charge in a controlled manner.
Prior art FIGS. 3-5 illustrate one conventional method of discharging accumulated charge from the implantation workpiece. Prior art FIG. 3 is a plan view of an implantation pedestal 30 having one or more grounded, conductive electrodes 32 formed on a surface 34 thereof. Prior art FIG. 4 is a cross sectional view of the pedestal of prior art FIG. 3, taken along dotted line 4xe2x80x944. The conductive electrodes 32 are grounded and thus readily provide a discharge path for any charge which may accumulate on the workpiece during ion implantation, thereby preventing a substantial accumulation of charge on the workpiece and protecting the workpiece from damage associated with an uncontrolled discharge.
As illustrated in prior art FIG. 4, the conductive electrodes 32 are formed on the pedestal surface 34 and thus represent raised portions which impact the topography of the pedestal 34. More particularly, the raised conductive portions 32 negatively impact the planarity of the pedestal surface 34. A resultant condition is illustrated in prior art FIG. 5, wherein the wafer or workpiece 16 overlies the pedestal surface 34 and makes contact with the grounded, conductive electrodes 32. Note that due to the conductive portions 32 being raised, the wafer 16 does not make good thermal contact to the chuck surface 34. As is generally known in the art, ion implantation, particularly in serial batch type systems, causes a significant amount of workpiece heating. The poor thermal contact caused by the electrodes 32 may cause the workpiece to overheat during implantation, thereby causing thermal damage to the workpiece or negatively impacting the implantation characteristics.
In addition to the poor thermal contact caused by the conductive raised portions 32, the physical contact caused at the wafer/electrode interface may generate particulate contamination in the ion implantation chamber, which undesirably may result in defects. Therefore although the raised, grounded electrodes 32 of FIGS. 3-5 are an effective means of discharging positive charge that would otherwise accumulate on the workpiece during implantation, the prior art solution negatively impacts the effective conduction of heat away from the workpiece and may contribute to increased particulate contamination.
Therefore there is a need in the art for an apparatus and method of addressing charge accumulation on the wafer during ion implantation which overcomes the disadvantages associated with the prior art.
The present invention is directed to an electrostatic clamp having a dielectric layer and a conductive portion which is associated and generally coplanar therewith. The coplanarity of the conductive portion with the dielectric layer surface allows for a grounding thereof for bleeding accumulated charge off of a workpiece situated thereon during processing while concurrently maintaining thermal contact with the workpiece and avoiding substantially the generation of particulate contaminants.
According to one aspect of the present invention, an electrostatic clamp is disclosed in which a dielectric layer overlies an electrode. The dielectric layer has a doped region embedded in a top surface thereof opposite the electrode. The doped region is electrically conductive while the remaining portion of the dielectric layer is electrically insulative. The doped region, when coupled to a circuit ground or other discharge potential, provides a discharge path for charge that accumulates on a workpiece (that rests on the top surface of the dielectric layer) during processing, such as ion implantation. Consequently, surface charge which would otherwise accumulate on the workpiece and create a potential reliability issue due to uncontrolled discharge, is discharged during workpiece processing in a controlled manner.
According to another aspect of the present invention, the electrode which underlies the dielectric layer, when a predetermined electric potential is applied thereto, exerts an electrostatic coulombic force on the workpiece, thereby securing the workpiece to the top surface of the dielectric layer without additional contact to the workpiece necessary. The doped region occupies only a portion of the top surface of the dielectric layer, therefore providing adequate area for removal of charge from the workpiece without affecting or otherwise interfering substantially with the electrostatic clamping force exerted on the workpiece via the underlying electrode.
According to still another aspect of the present invention, the dielectric layer or electrically insulating layer comprises a polyimide material such as KAPTON. The doped region in the polyimide exhibits broken or otherwise altered chemical bonds or structure due to the implantation of dopant therein. The broken bonds are believed to increase delocalized xcfx80-electrons and enhance charge mobility due to chain cross-linking. In any event, the doped regions exhibit a substantial electrical conductivity, for example, about 3xc3x9710xe2x88x923 xcexa9-cm, compared to the undoped regions of the polyimide which may exhibit a resistivity of about 1013 xcexa9-cm.
According to yet another aspect of the present invention, the dielectric layer, for example, a plastic material, a ceramic material or a glass material, is selectively doped with a conductive material such as a metal at a top surface thereof to generate an electrically conductive doped region therein. The electrically conductive doped region or regions, when coupled to a circuit ground or other type discharge potential, for example, provides a discharge path for charge which accumulates on the workpiece during processing thereof.
The coplanarity of the doped region within the dielectric layer, whether conductive due to broken chemical bonds or by doping with a conductive material, enables good thermal contact between the workpiece and the dielectric layer, thereby facilitating effective thermal conduction of heat away form the workpiece during processing such as ion implantation, and maintaining the workpiece in a generally safe or otherwise acceptable temperature range. In addition, the coplanarity of the top surface of the electrostatic clamp reduces particulate contamination associated with some conventional type electrostatic clamps.
According to yet another aspect of the present invention, a method of forming an electrostatic clamp is disclosed. The method comprises forming an electrically insulating material or layer over a conductive electrode. The electrically insulating material is then selectively doped to form a doped region which is generally coplanar with a top surface thereof. The doped region is electrically conductive and when coupled to a circuit ground potential, for example, provides a controlled discharge path to remove charge which would otherwise accumulate on the workpiece undergoing processing.
To the accomplishment of the foregoing and related ends, the invention comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.